Atrial arrhythmias and supra ventricular tachycardias, such as atrial fibrillation, atrial flutter and atrio-ventricular re-entry, are common post-operative complications among heart surgery patients. It is estimated that during the first seven to ten days after cardiac surgery post-operative supra ventricular tachycardia occurs in up to 63 percent of patients. Aranki et al. showed that patients with postoperative atrial fibrillation have a mean hospital stay of about fifteen days, whereas those patients without post-operative atrial fibrillation have a mean hospital stay of about ten days. Whether such extended hospitalization stays are primarily caused by arrhythmias is not known. See Cardiac Surg. Kirklin J W, Barrat-Boyes BC (Eds.): NY 1993, pg. 210, "The Importance of Age as a Predicator of Atrial Fibrillation and Flutter after Coronary Artery Bypass Grafting", Leitch et al., J. Thorac. Cardiovasc. Surg., 1990:100:338-42; "Atrial Activity During Cardioplegia and Postoperative Arrhythmias", Mullen et al., J. Thorac. Cardiovasc. Surg., 1987:94:558-65.
The presence of such arrhythmias, which in otherwise healthy patients may not be unduly serious, may be especially harmful to heart surgery patients. The surgery itself, the effects of prolonged anesthesia, or both have often already compromised the hemodynamic condition of such patients. Drugs that might be used to prevent post-operative atrial fibrillation are often only partially effective and may have negative effects on cardiac pump function.
Supra ventricular tachycardias may further cause a very irregular ventricular rate, which in turn can lead to hemodynamic conditions deteriorating even further. Such deterioration is especially serious for patients having a compromised left ventricular function. Such complications may also present a serious impediment to the recovery of the patient. See, for example, "Maintenance of Exercise Stroke Volume During Ventricular Versus Atrial Synchronous Pacing: Role of Contractility", Ausubel et al., Circ., 1985:72(5):1037-43; "Basic Physiological Studies on Cardiac Pacing with Special Reference to the Optimal Mode and Rate After Cardiac Surgery", Bailer et al., Thorac. Cardiovasc. Surg., 1981:29:168-73.
Due to the serious and potentially life threatening nature of the foregoing conditions, post-operative treatment is often aimed at preventing arrhythmias, such as through the use of drugs. Drugs, however, have been found not always to be effective at preventing arrhythmias. Thus, it is often necessary to provide a means for terminating any arrhythmias, which may occur. One common such means is over-pacing, more about which we say below.
If post-operative atrial fibrillation proves to have unacceptable hemodynamic consequences or causes serious symptoms, and if it does not stop spontaneously or antiarrhythmic drugs are ineffective in treating it, external cardioversion or atrial defibrillation may be required. But external atrial defibrillation, although generally effective as a treatment, may have profound side effects. First, and in contrast to ventricular defibrillation where conversion to normal sinus rhythm may occur after the first shock, atrial defibrillation may not be obtained until after several shocks have been delivered to the patient. This is because ventricular contraction continues during supra ventricular tachycardia. Due to the large amounts of energy, which must be delivered in external defibrillation (e.g., 40 to 360 Joules), the shocks are not tolerated well by conscious patients. External defibrillation is therefore preferably performed under general anesthesia or at least when the patient is sedated. The use of anesthesia gives rise to yet another patient risk factor.
External defibrillation requires relatively high energy because the electrical source is not positioned directly upon the cardiac tissue and instead must pass through the thorax, which tends to dissipate the energy. In contrast, internally applied atrial defibrillation, such as may occur during surgery through defibrillation paddles placed directly on the heart, requires considerably less energy because the defibrillation electrical energy is applied only to the tissue that needs to be defibrillated. In fact, direct atrial defibrillation may be accomplished with only one-Joule pulses in contrast to the 40 Joule and greater pulses required for external defibrillation. See, for example, Kean D., NASPE abs. 246, PACE, April 1992, pt. II, pg. 570.
Defibrillation success rates generally depend on the amount of energy delivered. The lower amount of energy delivered, the lower the defibrillation success rate and the greater the number of shocks that must be applied to obtain successful defibrillation. By way of contrast, in direct atrial defibrillation, where energy is applied directly to the heart, the energy level can be selected such that the patient may more easily tolerate both the amount of energy delivered as well as the number of shocks required.
Waldo et al. in "Use of Temporarily Place Epicardial Atrial Wire Electrodes For The Diagnosis and Treatment of Cardiac Arrhythmias Following Open-Heart Surgery," J. Thorac. Cardiovasc. Surg., 1978, vol. 76, no. 4, pp. 558-65 disclose the use of a pair of temporary heart wires placed on the atrium to diagnose and treat arrhythmias through anti-tachycardia overdrive pacing. Specifically, temporary heart wires were sutured to the atrial walls at the time of the heart surgery. Once the patient was ready to be released from hospital, the wires were removed by traction or pulling upon the external end.
Temporary post-operative atrial and ventricular pacing with temporary heart wires has been found to successfully treat many post-operative arrhythmias. As such, the procedure has become widespread--at least 100,000 such procedures are performed each year. Several problems, however, were encountered in the system disclosed by Waldo et al., referred to above. One problem encountered was the instability of heart wires mounted within the atrial wall. Because the wall undergoes constant motion, temporary heart wire leads were found to dislodge more often than was acceptable. Secondly, the relatively thin atrial walls (especially in elderly patients) were sometimes torn when the leads were removed by traction means.
An improved method of temporarily affixing heart wires onto the atrium was achieved with the introduction of the Medtronic Model 6500 Temporary Myocardial Pacing Lead System. That lead system featured a silicone atrial fixation disk to fasten the lead to the atrium, where the silicone atrial fixation disk was permanently sutured to the atrium. The lead was positioned so that it was trapped between the disk and atrial tissue. The lead could then be removed by simply pulling it from between the disk and the tissue. The rubber disk remained in the body after removal of the electrodes. The advantages offered by such a fixation system included more reliable lead fixation along with protecting the relatively thin atrial walls from tearing during lead removal. Thus, the Medtronic Model 6500 Temporary Myocardial Pacing Lead permitted post-surgical temporary anti-tachycardia over-drive pacing to be performed more safely.
Temporary anti-tachy overdrive pacing is not, however, always effective in terminating postoperative atrial arrhythmias or supra ventricular tachycardias. When drugs and overdrive pacing do not prevent or terminate such arrhythmias and tachycardias, or when inotropic drug side effects are contra-indicated, it may become necessary to perform atrial defibrillation, synchronized to the R-wave of the electrogram, to terminate potentially life-threatening arrhythmias. Because of the large energies required in defibrillation, however, many prior art temporary heart wires are not suitable for such applications.
In response to the foregoing concerns and problems in the prior art, a first generation Model No. 13004 Temporary Atrial Patch Electrode (TAPE) lead was developed by MEDTRONIC, INC. as further described in U.S. Pat. No. 5,527,358 entitled "Temporary Medical Electrical Lead" to Mehmanesh et al. The Model No. 13004 was developed as a clinical research device to investigate the feasibility of reducing post-operative atrial defibrillation in patients who underwent cardiac surgery and who were at risk of developing post-operative atrial fibrillation. TAPE electrodes were placed on the free walls of the left and right atrium. The TAPE lead comprised a defibrillation lead featuring a polytetrafluoroethylene (PTFE or TEFLON) felt pad onto which three parallel stainless steel defibrillation wire electrodes were mounted. The primary purpose of the TEFLON electrode mounting pad was to reliably fix the defibrillation lead to the atrium and to protect the atrial wall from sustaining electrical damage. In a clinical setting the Model No. 13004 TAPE lead demonstrated a Mean Defibrillation Threshold (DFT) of 1.5 Joule at 160 Volts in 23 patients. The implant time per device averaged only about 3 minutes to about 5 minutes. The Model No. 13004 TAPE lead was demonstrated to be successful in terminating post-operative atrial fibrillation.
One problem discovered in using the Model No. 13004 TAPE lead was that following removal of the defibrillation electrode and lead from the atrial wall, the TEFLON electrode mounting pad remained implanted permanently within the patient, potentially leading to an increase in the risk of infection. Additionally, it was discovered that in some patients the permanently implanted electrode mounting pad might become encapsulated by dense fibrous tissue, which in turn could lead to stiffening of the atrial wall.